This invention relates to an identification card comprising a partially light reflective retardation device. Said identification card is tamper-free and can be manufactured at low cost using techniques known in the art.
Identification cards such as so-called credit cards or security cards bearing information relating to the identification of the card holder have been used for a long time and various types have been proposed or used in commerce. Many of the earlier types of such identification cards had a serious disadvantage that they could be altered without telltale signs of alteration. In order to rectify the above-mentioned disadvantage of the earlier types of identification cards and thereby enhance the security aspect of such identification cards, various identification cards have been proposed in the past which comprise a means for detecting the alteration of the information or indicia contained in the card. Various physical or chemical principles have been utilized for detecting alterations of the card.
The present invention relates to a novel, tamper-free identification card which utilizes light polarization effect as a means for detecting fraudulent alterations of the card. Said light polarization effect is produced by a partially reflective retardation device. Thus, the identification card of this invention comprises an information-bearing substrate made of, for instance, a flexible but substantially rigid material, a partial reflector superposed over said substrate and a substantially transparent 90.degree. retarder superposed over said retarder. Said retarder/reflector combination which constitutes a partially reflective retardation device is sufficiently transparent so that the information contained in said substrate is visible to the eye through said retarder/reflector combination but at the same time is sufficiently reflective to produce the light polarization effect described below in more detail so that alterations of the identification card can readily be detected. Said retarder/reflector lamination is substantially transparent in both the altered and unaltered states, but when viewed through a detection device comprising, for instance, a linear light polarizer, the sign of the alteration of the card becomes immediately obvious as a color contrast between the altered and unaltered areas of the card. The identification card of this invention can be manufactured at a low cost and it does not require any expensive detection device.